1. Field of the Invention
This invention relates to a space transmission optical communication system, particularly to an optical communication system which is suited for transmitting optical signals to a moving station located on a vehicle.
2. Description of the Prior Art
In conventional optical communication systems which are most widely used, a transmitting station and a receiving station are connected by an optical-fiber cable through which optical signals converted from data signals are sent from the transmitting station to the receiving station at which the data signals are reproduced from the received optical signals. Optical transmission systems of this type are advantageous in that the data transmitted through optical-fibers are free from electromagnetic induction that could be caused by high-tension cables, thunder-lightning and so on, and that they suffer little from transmission loss.
Meanwhile, space transmission optical communication systems that deliver optical signals through space without using optical-fiber cables have been also proposed. With such space transmission optical communication systems, a transmitting station modulates high-frequency carrier waves, using data signals, into modulated waves and sends out the obtained modulated light, while a receiving station receives the transmitted modulated light and photoelectrically convert and demodulate it into data signals.
Some optical communications systems require that the receiving station should be mobile, such as the one located on a vehicle. It is of course that the aforementioned cable transmission is not applicable to the mobile station. Further, to transmit adequate data to a vehicle that passes through the effective transmittable region of the delivered signals at high speed, it is necessary to increase the transmission rate of data. But, because the circuit used in the conventional space transmission optical communication systems responds to the high-frequency carrier waves, the transmission rate of data cannot be increased to the limit of the response speed of the circuit. To further increase the transmission rate of data, a circuit that could respond to signals of still higher frequencies should be designed. Such circuits will be intricate and costly. Also, in the space transmission optical communication system, attenuation and disappearance of optical signals in transmission space due to their directionality and external disturbing factors present serious problems. A fog in the transmission can greatly weaken the intensity of optical signals. When sunlight directly hits the receiving unit of the receiving station, optical signals can disappear under the influence of its energy. The directionality of optical signals can be a serious problem, too; especially so with the mobile station located on a vehicle. Optical signals of high intensity with high-precision directionality should be sent out to the vehicle running at high speed.